Fluid transport may be characterized based on the mechanism that causes flow within the device. When fluid transport pertains to a nonspontaneous fluid flow regime that, for the most part, is the result of a force external to the device, the fluid transport is considered "active". On the other hand, when the fluid transport pertains to a spontaneous flow regime that stems from a property inherent to the device, the fluid transport is considered "passive". A catheter is a well-known example of an active fluid transport device. Typically, catheters are connected to a vacuum source that draws liquid through the device. An example of a passive fluid transport device is an absorbent pad or sponge.
The design of an active fluid transport device depends largely on the specific application to which it is applied. In many cases, it is desirable to control the fluid flow path. In one sense, the fluid flow path can be controlled for the purpose of running a particular fluid near another object or fluid such as in a heat exchanger.
A heat exchanger that has discrete microchannels extending between first and second manifolds is described in U.S. Pat. No. 5,317,805 to Hoopman et al. This microchanneled heat exchanger is produced by materially depositing a shell--such as by electroplating or vapor depositing a metal--about a sacrificial core. Another microchanneled heat exchanger is described in U.S. Pat. No. 5,070,606 also to Hoopman et al., which is made by forming a plastic or ceramic body about an arrangement of fibers that are subsequently removed to leave microchannels within the formed body.
In another sense, a fluid flow path can be controlled to make the fluid flow according to specific flow characteristics. That is, fluid flow may be facilitated simply through a single conduit, between layers, or by way of multiple channels. Other examples include porous products that can be attached to a potential generating device that causes fluid flow through the porous product--see, for example, U.S. Pat. No. 5,599,330 to Rainin, U.S. Pat. No. 4,533,352 to Van Beck et al., and U.S. Pat. No. 3,935,863 to Kliger.
A fluid transport flow path may be defined by multiple channels that are employed to transport a liquid from a collection site to another location such as a storage receptacle. When using such devices quite often, the liquid mixes with gas and a two-phase flow results, sometimes as a turbulent mix of the liquid and gas. Depending on mix ratios and the velocity of the fluid stream, the liquid and gas can combine in undesirable flow patterns. Patterns known as froth, dispersed, and slug flow can sometimes detrimentally affect the fluids being conveyed, or such flow can affect the transport system and/or surrounding environment.
In froth flow, for instance, bubbles of gas are dispersed throughout the liquid, and intimate contact of the air and liquid could cause, as an example, accelerated oxidation of the liquid. In dispersed flow, nearly all the liquid is entrained as fine droplets in the gas. Aerosol droplets generated by this flow pattern could find their way into the local environment, thus creating a hazard depending on the nature of the liquid. An example might be in a surgical field where bio-contaminated fluids, aspirated for disposal, become aerosolized and enter into work environment when collection canisters are opened. In slug flow, a wave of liquid is picked up periodically by rapidly moving gas to form a frothy slug that passes along the system at a velocity greater than the average liquid velocity. In this type of flow, slugs can cause equipment vibration when impacting upon system components. The impact can place a high level of mechanical stress on the fluid, in addition to elements in the system such as fittings and bends. Violent fluid transport action also could possibly break down the cellular structure of stress sensitive fluids, such as blood that is being collected for reintroduction during surgical procedures.
Two-phase flow can also detrimentally impact the workplace by generating noise during the fluid transport. That is, noise is often created when a turbulent liquid mixture moves through the device. Noise pollution must be minimized in many environments, particularly where good communication is essential, such as in an operating room.
U.S. Pat. No. 4,966,584 to Nguyen describes an active fluid transport device that addresses noise concerns. Specifically, a suction aspirator is described that is used during surgical procedures. The device incorporates a valve assembly that controls flow through the device to reduce noise. The valve purportedly controls noise by regulating suction flow through the device.
Another active fluid transport device that is used in the medical field is a fluid recovery floor mat that is commercially available under the trade name "Fluid Control" floor suction mat, from Technol Medical Products Inc. This product uses both passive and active means to remove fluids that fall from a surgical site during a surgical procedure. The device has an absorbent mat that resides above a multitude of parallel channels. Holes are provided in the channel surfaces that interface with the absorbent mat so that fluid recovered by the mat can be drawn into the channels. The parallel channels are connected to a manifold that attaches to suction tubing. Thus, after fluid has accumulated on the mat, removal can be facilitated through the multiple channels by applying a vacuum. Because the recovered liquid can flow through the channels as a liquid/air mixture, the device has the potential to generate noise. That is, as fluid is pulled through the channels by the suction system, air can mix with the liquid to generate noise that can negatively affect the working environment.
U.S. Pat. No. 5,628,735 to Skow discloses an active fluid transport device that professes to gently and continuously remove unwanted fluid from an operating field during surgery. The device utilizes both passive and active fluid transport mechanisms to remove excess fluids during surgical procedures. It includes a flexible mat that has a high wicking property, and embedded in the mat is a flexible suction tube that is attached to a suction source and that removes fluid from the mat to prevent it from becoming saturated. The tube has one or more holes in it to allow the recovered fluid to flow from the mat into the tube. Because the device does not employ multiple discrete channels, it is not capable of distributing the suction effect amongst such channels, and the use of a tubing can restrict applications for the device.
Other active fluid transport products that are used to recover fluids are described in U.S. Pat. Nos. 5,437,651 to Todd et al. and 4,679,590 to Hergenroeder. In Todd et al., a medical suction apparatus is disclosed that is useful for collecting blood and other fluids that accumulate in a patient during surgical procedures. The apparatus includes an absorbent foam pad that is attached to a suction source via a flexible backing plate. The backing plate has channels that direct the recovered fluid toward an orifice. This apparatus is highly susceptible to generating noise because a liquid/air mixture would commonly be drawn through the apparatus.
Hergenroeder describes a receptacle for collecting fluid from the floor of an operating room. The device is described as being especially suitable for collecting irrigation fluids used in surgical procedures such as arthroscopic surgery on a joint, e.g., a knee. The receptacle is thin and generally flat and has a gridwork of small basins that form a collecting surface with drains through which the recovered fluid flows to channels formed between the receptacle and the floor. The channels direct the fluid to a common discharge port, which may be connected to a suction device. Because of the channel configuration, this device, like others described above, may easily create a two-phase liquid/air flow, which could generate significant noise pollution.
Other flexible tubing devices or catheters are described, for example, in U.S. Pat. Nos. 5,445,771 to Degen, 4,623,329 to Drobish et al., and 3,598,127 to Wepsic. In the Degen patent, a collection of hollow thermoplastic fiber strands are secured together and used in the way of a separatory device. The devices described in Drobish et al and Wepsic are directed to catheters that have a primary flow passage and one or more secondary flow passages disposed between tubing layers. The Drobish et al device includes a concentric secondary passage that has grooves formed within one of the defining surfaces for increasing surface area that is accessible to fluid passed within the secondary passage. In the Wepsic patent, plural secondary v-shaped grooves are arranged around a primary passage. The v-shaped grooves are enclosed by an outer tube that is permeable to an antibacterial substance.